Reference to a Satellite Positioning System or SATPS herein refers to a Global Positioning System (GPS), to a Global Orbiting Navigation System (GLONASS), and to any other compatible satellite-based system that provides information by which an observer""s position and the time of observation can be determined.
The Global Positioning System (GPS) is being developed and operated to support military navigation and timing needs at an estimated cost of about $8-10 billion. GPS represents an almost ideal dual-use technology and enjoys increased attention by civilians to explore its suitability for civil applications. The complete GPS system consists of 24 operational satellites and provides 24-hour, all-weather navigation and surveying capability worldwide. A major milestone in the development of GPS was achieved on Dec. 8, 1993, when the Initial Operational Capability (IOC) was declared as 24 satellites were successfully operating.
The implication of IOC is that commercial, national, and international civil users can rely on the availability of the Standard Positioning Service. Current policies quantify SPS as 100-meter, 95% position accuracy for a single user. Authorized (military) users will have access to the Precise Positioning Service (PPS), which provides a greater degree of accuracy. The PPS access is controlled by cryptographic means.
The GPS satellites transmit at frequencies L1=1575.42 MHz and L2=1227.6 MHz modulated with two types of codes and with a navigation message. The two types of codes are the C/A-code and the P-code. SPS is based on the C/A-code, whereas PPS is provided by the P-code portion of the GPS signal. The current authorized level of SPS follows from an intentional degradation of the full C/A-code capability. This measure is called selective availability (SA) and includes falsification of the satellite clock (SA-dither) and the broadcast satellite ephemeris (SA-epsilon), which is part of the navigation message. Despite selective availability, the C/A-code is fully accessible by civilians. On Jan. 31, 1994 the SA was finally implemented. The purpose of SA is to make the P-codes available only to authorized and military users. Users should be equipped with a decryption device or the xe2x80x9ckeyxe2x80x9d in order to lock on to P-codes. SA is implemented through a modification of the mathematical formula of the P-code using a classified rule. The encrypted P-code is referred to as the Y-code.
Two types of observables are of interest to users. One is the pseudo-range, which equals the distance between the satellite and the receiver plus small corrective terms due to clock errors, the ionosphere, the troposphere, and the multipath. Given the geometric positions of the satellites (satellite ephemeris), four pseudo-ranges are sufficient to compute the position of the receiver and its clock error. Pseudo-ranges are a measure of the travel time of the codes (C/A, P, or Y).
The second observable, the carrier phase, is the difference between the received phase and the phase of the receiver oscillator at the epoch of measurement. Receivers are programmed to make phase observations at the same equally spaced epochs. The receivers also keep track of the number of complete cycles received since the beginning of a measurement. Thus, the actual output is the accumulated phase observable at preset epochs.
(The above-referenced discussion is provided in the book xe2x80x9cGPS Satellite Surveyingxe2x80x9d, Second Edition, authored by Alfred Leick, and published by John Wiley and Sons, Inc. in 1995; pp 1-3).
Both the SPS and PPS address xe2x80x9cclassicalxe2x80x9d navigation, where just one receiver observes the satellites to determine its geocentric position. Typically, a position is computed for every epoch of observation.
However, in the surveying and geodesy applications the relative or differential positioning is used, wherein the relative location between the receivers is determined. In this case, many of the common mode errors cancel or their impact is significantly reduced. This is particularly important in the presence of selective availability.
The multipath errors originate with contamination of SATPS signals by delayed versions of these signals. For some applications using either pseudo-range carrier phase observables, multipath is the dominant error source. The most direct approach for reducing this error is to select an antenna site distant from reflecting objects, and to design antenna/back plane combinations to further isolate the antenna from its surroundings. In some cases, however, antennas should be located in relatively poor sites, and other techniques for code multipath reduction are required.
In the U.S. Pat. No. 5,414,729, issued to Fenton, a receiver for pseudorandom noise (PRN) encoded signals is disclosed.
The Fenton receiver consists of a sampling circuit, multiple carrier and code synchronizing circuits, and multiple correlators, with each correlator having a selectable code delay spacing. The time delay spacing of the multiple correlators is distributed around an expected correlation peak to produce an estimate of the correlation function parameters which vary with respect to multipath distortion. This information may be used in turn to determine the offset estimates for locally generated PRN reference code and carrier phase tracking signalsxe2x80x9d, Col. 3, lines 21-36. In another embodiment of the Fenton device, xe2x80x9cthe majority of the channels in a receiver can be left to operate normally, with one or more of the channels being dedicated to continuously sequencing from channel to channel to determine the multipath parameters for a partial PRN code being trackedxe2x80x9d, Col. 3, lines 44-49.
Thus, the Fenton device includes a plurality of tracking satellite channels used to estimate the multipath parameters.
However, the Fenton device ""729 includes a number of limitations.
(1) The Fenton device does only tracking. After tracking, there is still a multipath blimp left in the correlation function that has to be removed by some other means.
(2) The Fenton device uses the full correlation. Indeed, the Fenton device correlates through the entire chip period.
(3) The Fenton device employs multiple correlators time delay spacing. This is equivalent to Early minus Late (Exe2x88x92L) response function that is limited to one chip time period. That is, the Fenton device is a xe2x80x9cnarrow correlatorxe2x80x9d device. If Fenton""s (Exe2x88x92L) response function is not limited to only one chip time period and is extended over two chip periods, the Fenton device would not be beneficial over the prior art at all.
(4) Using the filter function approach, the xe2x80x9cnarrow correlatorxe2x80x9d property of the Fenton device can be described as follows: the amplitude of the Fenton""s filter function decreases when time increases between zero and two chip time periods.
Another technique for code multipath reduction, that is free of Fenton""s limitations, was disclosed by Rayman Pon in the U.S. patent application Ser. No. 08/650,631, entitled xe2x80x9cSuppression Of Multipath Signal Effectsxe2x80x9d (patent application #1), that was assigned to the assignee of the present patent application, and that was filed on May 20, 1996 and now patented U.S. Pat. No. 5,903,597. The patent application #1 is specifically referred to in the present patent application and is incorporated herein by reference. In the patent application #1 the weighted tracking process was used in order to suppress the multipath error signal, wherein the Early and Late signals were non-uniformly weighted in order to suppress the multipath error signal.
One more example of such technique for code multipath reduction was disclosed by Rayman Pon in the U.S. patent application Ser. No. 08/783,616 entitled xe2x80x9cCode Multipath Reduction Using Optimized Additional Signalsxe2x80x9d (patent application #2) filed on Jan. 14, 1997. The patent application #2 assigned to the assignee of the present patent application, is specifically referred to in the present patent application and is incorporated herein by reference. In the patent application #2 the modified tracking process was used for the purposes of the multipath error signal suppression, wherein the Early and Late signals were modified to suppress the multipath error signal.
The code multipath reduction was based on the utilization of weighted correlation means (in application #1) or modified correlation means (in application #2) that changed the magnitude and shape of the composite signal autocorrelation function to suppress the contributions of a multipath signal. However, although the multipath signal is suppressed, the residual multipath component signal was still present in the composite signal.
One technique for estimation and minimization of the residual code multipath error signals for small delays was disclosed by Rayman Pon in the U.S. patent application Ser. No. 08/683,859 entitled xe2x80x9cCode Multipath Error Estimation Using Weighted Correlationsxe2x80x9d (patent application #3) that was assigned to the assignee of the present patent application, and that was filed on Jul. 19, 1996 and now patented U.S. Pat. No. 5,966,403. The patent application #3 is specifically referred to in the present patent application and is incorporated herein by reference.
The patent application #3 discloses the invention that is very different from the Fenton patent.
(1) Indeed, in the patent application #3 only code tracking is performed. However, a multipath blimp left in the correlation function after code tracking can be removed by using an additional circuitry, as disclosed in the current patent application. See discussion below.
(2) The patent application #3 employs a partial weighted correlation function that is partially non-zero during one chip time period.
(3) The patent application #3 employs the xe2x80x9cbroad correlatorxe2x80x9d approach including an (Exe2x88x92L) response function that is not limited to one chip time period and can be extended up to two chip time periods.
(4) Using the filter function approach, the xe2x80x9cbroad correlatorxe2x80x9d property of the device of the patent application #3 can be described as follows: the amplitude of the filter function of the patent application #3 is flat and does not change when time increases between zero and two chip time periods.
In the present patent application one additional technique for estimation and minimization of the residual code multipath error signals for small delays is disclosed. The current patent application employs the xe2x80x9cbroad correlatorxe2x80x9d approach.
One aspect of the present invention is directed to an apparatus for use in decoding a composite signal, wherein the composite signal includes a signal from a transmitter and a distortion component.
In one embodiment, the transmitter includes a GPS satellite system. In this embodiment, the distortion signal includes a multipath error signal.
The apparatus includes a receiving circuit configured to receive the composite signal and at least one additional circuit. The code receiving circuit generates a code receiving function having a residual multipath error response envelope. Each additional circuit is configured to generate an additional signal used to estimate and minimize the residual multipath error response envelope.
There are three basic embodiments for the receiving circuit: weighted, modified, and mixed.
In each basic embodiment, the receiving circuit further comprises at least two partial receiving circuits.
In the weighted embodiment, at least one partial code receiving circuit further comprises a weighted partial receiving circuit configured to provide a satellite partial code receiving function with reduced multipath error.
In the modified embodiment, at least one partial code receiving circuit further includes a modified partial receiving circuit configured to provide a satellite partial code receiving function with reduced multipath error.
In the mixed embodiment, one partial code receiving circuit has a weighted embodiment and one partial code receiving circuit has a modified embodiment.
Each weighted or modified partial code receiving circuit can have five additional embodiments.
In the first weighted (or modified) embodiment, at least one partial receiving circuit further comprises an (I and Q) input weighted (or modified) partial circuit configured to process the input composite signal from the satellite and to generate a weighted (or modified) partial component of the input (I and Q) composite signal.
In the second weighted (or modified) embodiment, at least one partial receiving circuit further comprises an (I and Q) weighted (or modified) partial local carrier reference circuit configured to generate an (I and Q) weighted (or modified) partial component of a local carrier reference signal.
In the third weighted (or modified) embodiments, at least one partial receiving circuit further comprises a complex mixer weighted (or modified) partial circuit configured to generate an inphase (I) weighted (or modified) partial component of a baseband sampled composite signal.
In the fourth weighted (or modified) embodiment, at least one partial receiving circuit further comprises a Local code weighted (or modified) partial reference circuit configured to locally generate a Local code weighted (or modified) partial reference signal.
In the fifth weighted (or modified) embodiment, at least one partial receiving circuit further comprises a Local weighted (or modified) partial code correlation circuit configured to generate a Local weighted (or modified) partial component of a composite correlation signal.
Each additional circuit can be implemented in five weighted or in five modified embodiments in the same way as a partial receiving circuit can be implemented.